Be sure to do the following:

Feb 24, 2009 at 12:45 AM
Locate the Web.Config file under the InetPub virtual directory(s) you currently have SharePoint deployed to and add this line in the SafeControls section:

<SafeControl Assembly="LookupWithPicker, Version=, Culture=neutral, PublicKeyToken=e1eb4ce568bda77d" Namespace="LookupWithPicker" TypeName="*" Safe="True" />

^^ The PKT may be different on your server , to check it just go to the Windows/Assemblies and look to see what it is and replace the number accordingly ^^

be sure to do an IISRESET.

Also - Add this to each front-end web in your farm as needed.
Sep 4, 2009 at 8:20 AM

Yes, this must be done in order to get it work.

I think the author should include it in the documentation or in install.bat.